1. Field of the Invention
The present invention relates to an echo processing method and device, and in particular to a method and device for processing a line echo which occurs in a converter between a digital line and an analog line in a private branch exchange (PBX), a conference device, and the like.
2. Description of the Related Art
In order to make communications between a digital telephone and an analog telephone, a conversion between a digital 4-wire line and an analog 2-wire line is required.
FIG. 10 shows a 2-wire/4-wire converter used for such a conversion. In the 2-wire/4-wire converter, due to impedance mismatching, a near end signal from a digital line of an extension line, VoIP, or the like on a transmitting side turns around to a receiving side as a line echo LE as shown in FIG. 10. It is known that a near end receiver unintentionally hears a voice of a near end caller with a time delay due to the line echo LE, which leads to interference of the conversation.
As a technology for cancelling such a line echo, an echo canceller has been well known. One example of the echo canceller is mentioned in the patent document 1. As shown in FIG. 11, by performing a convolution to a reference signal (transmitting signal Ts) that is a near end signal As in an adaptive filter 5_1 composed of a digital filter, an estimated echo signal (pseudo echo signal) Ec is generated, so that an echo component is removed by subtracting the estimated echo signal Ec from a receiving signal Rs in a subtractor 5_2. Concurrently, with the output signal of the echo canceller 5 being made an error signal Es, filter coefficients of the adaptive filter 5_1 are adaptively updated so that the error signal Es may become small (learning of propagation characteristic).
If such a learning of propagation characteristic is performed while no error is occurring, an erroneous learning will be performed, resulting in a deterioration of an echo removal performance. Accordingly, an accurate detection of a section where an echo is occurring and a learning control based on the detection result become important.
As an echo detection system for detecting such an echo section, various systems have been proposed, and the typical arrangement thereof is shown in FIG. 12. In this echo detection system, an echo detector 6 is added to the echo canceller 5. The transmitting signal (reference signal) Ts, the receiving signal Rs, and the error signal Es of the echo canceller 5 are acoustically analyzed/compared as shown in the following (1)-(3), so that a detection result Dr of the echo section is outputted, only in which the echo canceller 5 is driven:
(1) Power difference between the transmitting signal Ts and the receiving signal Rs
(2) Echo cancellation amount (power difference between the receiving signal Rs and an output signal Os), or the variation thereof
(3) Cross-correlation value between the transmitting signal Ts and the receiving signal Rs
It is to be noted that there has been proposed an initialization type echo canceller which connects a signal source of an appropriate frequency to 4-wire transmitting points upon training, and obtains a gain coefficient and a phase coefficient by monitoring responses at four reception points, thereby enabling an initialization of the echo canceller to converge in a short time (see e.g. patent document 2).
Furthermore, there have been a double-talk detecting method and device which accurately detect presence/absence of double-talk even when a speech transmitting power of a far end speaker is larger than that of a near end speaker (see e.g. patent document 3).
Furthermore, there has been an echo canceller in which a suppression degree of the echo by the echo canceller provided in the 4-wire section of the 2-wire/4-wire converter of the private branch exchange is reduced depending on a length of a line of a 2-wire subscriber line, thereby reducing interference of communication (see e.g. patent document 4).    [Patent Document 1] Japanese Patent Application Laid-open No. 64-27325    [Patent Document 2] Japanese Patent Application Laid-open No. 63-236423    [Patent Document 3] Japanese Patent Application Laid-open No. 7-303072    [Patent Document 4] Japanese Patent Application Laid-open No. 9-312596
In the above-mentioned echo detection methods (1)-(3), there has been a problem that an echo detection accuracy is low due to the following effects:
(i) Effect of Far End Signal
Since the receiving signal includes a far end signal in addition to an echo, echo detection is erroneously made in some cases even if no echo occurs when the echo detection is performed by comparing the transmitting signal with the receiving signal (echo detection methods (1) and (3)).
(ii) Effect of Echo Delay
Since an echo arises due to the transmitting signal leaking into the receiving signal with a time delay, a correction of the time delay is required when the echo detection is performed by comparing the transmitting signal with the receiving signal (echo detection methods (1) and (3)). If the correction is erroneously performed, the echo detection itself may be erroneous.
(iii) Effect of Reduction of Echo Cancellation Amount
When the echo detection is performed based on the echo cancellation amount (or the time variation thereof) (echo detection method (2)), if the echo cancellation amount once decreases, the echo detection accuracy will also decrease, resulting in a vicious circle of inducing further decreases of the echo cancellation amount in some cases.